This invention relates to an inlet port for an internal combustion engine which has an inlet hole opened on the end face of a cylinder chamber at an off-center position, the inlet hole being provided with an inlet valve and connected to an inlet passage.
The inventors of the present invention already invented an inlet port for an internal combustion engine, as disclosed in U.S. patent application Ser. No. 531,391, wherein in order to generate a suitable level of stable swirl without reducing the volume efficiency, the inlet passage comprises a swirl generating section which is connected to the inlet hole and surrounds the inlet valve at the end section of the inlet passage and an introducing section which is the remaining section of the inlet passage excluding the end section, the width between the peripheral surface of the valve stem of the inlet valve and the wall surface of the swirl generating section is made larger at the outside part on the wall side of the cylinder chamber than at the inside part on the center side of the cylinder chamber, the introducing section is connected to the wider outside part of the swirl generating section, the height from the opening face of the inlet hole to the ceiling of the swirl generating section is constant or made to decrease from the wider outside part to the narrower inside part around the valve stem, and the average rate of reduction is set to not more than 0.18% of the inlet hole diameter per one degree around the valve stem. The prior invention can attain its aim intended, however, the strong swirl can not always be generated under the high volume efficiency.